Ken Kundert wrote on Sep 25th, 2006, 11:09am:There are two types of phase variation in oscillators. The dominant source of phase variation is due to oscillator phase noise, which results from noise coupling into the primary oscillation mode. In this case, the phase variation is slow and the same for all signals. In this case, it does not matter if you measure the phase variation at one point in the cycle or time averaged over the whole cycle, because the phase due to this source does not vary significantly over a cycle.
The second source of phase noise is due to sources that are isolated from the primary oscillation mode, such as noise from buffers or dividers. Noise due to these sources are usually modulated by the oscillation signal, in which case they can vary substantially over each cycle, and so should be characterized at the threshold if followed by a thresholding circuit.
And yes, switching mixers are thresholding circuits.
-Ken
Thanks for this very clear presentation, Ken. Unfortunately, when the people at Cadence implemented the pnoise jitter analysis for autonomous circuits, they seemingly forgot about the second source of phase noise you mention. So, instead of making the pnoise jitter analysis a superset of the pnoise timedomain analysis just like for driven circuits, they made it a superset of the pnoise modulated analysis (for those with SourceLink access, see
http://sourcelink.cadence.com/docs/db/kdb/2005/Oct/11196272.html). As far as I understand it, this has the consequence that only the first source of phase noise you mention is taken into account in the simulation. In the setup form for pnoise jitter analysis for autonomous circuits, you can specify neither a circuit node nor a threshold value. In my opinion, this way of implementing the analysis is not very useful because it might neglect important sources of jitter in an autonomous circuit.